1. Field of the Invention
The present invention relates generally to the field of energy recovery and power production systems and in particular to a black liquor or biomass gasifier combined with a diesel or dual-fuel engine and a boiler to recover chemicals and produce fuel and electrical power and high-pressure steam from the by-products of an industrial plant process, such as those of a paper mill.
2. Description of the Related Art
Environmental concerns and government regulations for pollutant levels in emissions of industrial plants have made the search for more efficient power generation necessary. The paper industry in particular has increased chemical recovery capacity and power generation needs due to changing processes designed to meet new and anticipated standards.
Electrical power requirements are often outpacing fossil fuel or steam generation requirements in newer paper mills. The increased electrical power needs arise from the addition of environmental control equipment and recycled, mechanical and semi-chemical pulp co-production processes.
Black liquor and biomass gasifiers for commercial use are currently being developed by several companies. Gasifiers can be used to process by-products, such as black liquor or biomass material into usable fuel gas. Conventionally, the clean gas produced by the gasifier components is used to drive gas turbines.
Several different gasifiers are described in the following publications: Finchem, K. J., "Black Liquor Gasification Research Yields Recovery Options for Future," Pulp & Paper, November 1995; Berglin, N., et al., "Energy System Options with Black Liquor Gasification," 1995 International Chemical Recovery Conference Pre-Prints, Vol. B, pp. B311-B315, CPPA, Montreal, Quebec, Canada, April 1995; Richard, J. C., "The 2020 Mill--Utility Requirements for Market Kraft Pulp," Tappi Journal, March, 1995, Vol. 78, No. 3, pp. 175-184; Bain, R. L., et al.,"New Gasification Technology Offices Promise for Biomass Plants," Power Engineering, August 1996, pp. 32-39; Consinni, S. Larson, "Biomass--Gasifier/Aeroderivative Gas Turbine Combined Cycles Part A--Technologies and Performance Modeling," Journal of Engineering for Gas turbines and Power, July 1996, Vol. 118, pp. 507-515.
These references describe combining a gasifier with a gas turbine to drive generators for producing electrical power. The use of gas turbines with gasifiers suffers from the requirement that high gasifier pressures are required to operate the turbines efficiently. The generated gas from the gasifiers must be thoroughly cleaned of contaminants such as particulates, tar, and alkali metals to avoid damage to the turbines. The turbine blades can erode, accumulate deposits or fail from excessive exposure to these contaminants. Turbine manufacturers have strict specifications for the allowable contaminant levels. Additional operating and maintenance procedures, and reduced turbine operating temperatures, and thus reduced turbine efficiency and output can result if higher levels of the contaminants are present. A higher fuel gas heating value per unit volume is required for current gas turbine designs than for dual-fuel engines and some gasifier designs operate below the limiting value for gas turbines. This would require co-firing with a significant quantity of higher quality auxiliary fuel to make use of a gas turbine feasible.
Diesel engines, or dual-fuel engines which use added diesel or distillate fuel to achieve compression ignition, have been used in power generation plants, burning gas, oil or lower quality gaseous fuels such as land-fill gas, mine gas, or gas from sewage digesters. Hereinafter "diesel engine" is intended to encompass both diesel engines and dual-fuel engines for purposes of this application. O'Keefe, W. "Engine Generators Reconfigured to Compete in the Next Century," Power, October 1995, pp. 52-62 generally describes the use of diesel or dual-fuel engines for electrical power generation.
Diesel engines have several advantages over gas turbines. Gas turbines generally produce large volumes of hot gas for steam generation. When steam production is not required, gas turbines which must still produce electrical power run at a lower cycle efficiency, and produce exhaust gas without having a process to use it. Diesel engines by contrast automatically adjust air flow during periods of decreased loading and eliminate this problem, remaining more efficient even at low loads. Diesel engines can operate using fuel gas having a higher level of contaminants than gas turbines allow. Production of pollutants such as NO.sub.x are also reduced by diesel engines operated in a dual-fuel configuration with very low levels of auxiliary fuel. Technologies to reduce pollutants from diesel engines to acceptable levels exist commercially. When combined with a boiler system and conventional gas cleanup, the level of NOx and SOx produced per unit of electrical power generated is lower than a comparable conventional design.